Carved from a hundred million pounds of stone, soaring atop a web of open masonry, gothic cathedrals are marvels of human achievement and artistry. But how did medieval architects and builders reach such spectacular heights? Here PBS television's NOVA investigates the hidden wonders of Europe's great cathedrals. Consuming the labor of entire towns, sometimes taking a hundred years to build, these engineering marvels were crafted using only hand tools. Centuries old, many now teeter on the brink of collapse; to save them, an international team of engineers, architects, art historians, and computer scientists are searching for clues to how they were originally built, and the obscure mathematics that made them possible.